Josh Radnor, aka Ted Mosby, has known the "big plot thing" since the start... but what is it?

After nine years, nine seasons and 208 episodes, the hit CBS sitcom, starring Jason Segel, Alyson Hannigan, Neil Patrick Harris, Cobie Smulders and Josh Radnor, is coming to a close. And all we can think about is: what's going to happen in the last ever episode?

The ninth season finally introduced the mother we've all been waiting for (played understatedly by Cristin Milioti) and we thought our happy ending was within reach, but as the season edges towards its forever finale, rumours are rife. Is the mother ill? Is the mother dead? What on earth is going on?

And while we fret and fuss, it turns out Josh Radnor, the guy behind Ted Mosby, has known the big secret all along. Since the show's inception back in 2005, in fact.

"I was let in on a big thing [in the] first season," Radnor recently revealed.

"I knew that one big plot thing early, but it wasn't something that I thought about so much as we were going on," he told The Hollywood Reporter. "I always tried to remember that the version of Ted I was playing was moving forward and didn't know everything. He wasn't the narrator with the benefit of hindsight. It was always better for me to just read the episodes each week and try to play him as honestly as I could each week. I trusted them that they were going to take him on the right journey."

Talking about the "big thing" in the finale, Radnor added: "I thought they weren't going to do that, and then they said, 'No, we're still doing that.'"

What that is, is still up for debate, and still will be until the final episode airs on Monday in the US.